Hybrid fiber-wireless networks incorporating WDM technology for fixed wireless access operating in the sub-millimeter-wave and millimeter-wave (mm-wave) frequency regions are being actively pursued to provide untethered connectivity for ultrahigh bandwidth communications. The architecture of such radio networks requires a large number of antenna base-stations with high throughput to be deployed to maximize the geographical coverage with the main switching and routing functionalities located in a centralized location. The transportation of mm-wave wireless signals within the hybrid network is subject to several impairments including low opto-electronic conversion efficiency, fiber chromatic dispersion and also degradation due to nonlinearities along the link. One of the major technical challenges in implementing such networks lies in the mitigation of these various optical impairments that the wireless signals experience within the hybrid network. In this paper, we present an overview of different techniques to optically transport mm-wave wireless signals and to overcome impairments associated with the transport of the wireless signals. We also review the different designs of subsystems for integrating fiber-wireless technology onto existing optical infrastructure.

Fiber-Wireless Networks and Subsystem Technologies

Advanced cost reduction and performance improvement techniques conceived for Radio Over Fiber (ROF) communications are considered. ROF techniques are expected to form the backbone of the future fifth generation of wireless networks. The achievable link performance and the associated deployment cost constitute the most salient metrics of an ROF architecture. In this paper, we commence by providing a rudimentary overview of the ROF architecture, and then we elaborate on ROF techniques designed for improving the attainable system performance. We conclude by describing the ROF techniques conceived for reducing the ROF system installation costs.

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Performance Improvement and Cost Reduction Techniques for Radio Over Fiber Communications

An optical carrier notch filter includes an optical coupler with at least first, second and third ports. The first port is configured to receive an output that includes an optical carrier and interleaved optical single sideband signals. An optical bandpass filter is coupled to a port of the optical coupler. The optical bandpass filter separates the output into a transmitted signal that contains the optical carrier, and a reflected signal that includes the interleaved optical single sideband signals. The reflected signal is reflected from the optical bandpass filter to the third port of the optical coupler.

Method and apparatus for interleaved optical single sideband modulation

To understand the transmission limitation of multiple narrow single-sideband subcarrier-multiplexed (SSB/SCM) signals, this paper first presents a closed-form analysis to predict Mach-Zehnder intensity modulator-induced composite triple beat (CTB), and linear-fiber-dispersion-induced composite second-order (CSO) and CTB distortions. To combine SSB/SCM with dense-wavelength-division-multiplexing (DWDM) systems, analytical and numerical tools, which are not constrained by any wavelength spacing and modulation frequencies, are used to analyze cross-phase modulation-induced crosstalk. All the analytical and numerical results are verified by computer simulations. Several multichannel SSB/SCM/DWDM systems with transport capacities of 10 or 20 Gb/s per wavelength, with a wavelength spacing of 25, 50, and 100 GHz, are also studied in this paper to understand the fundamental transmission limitations.

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Multichannel single-sideband SCM/DWDM transmission systems

We investigate a frequency up/down-converter based on a single cascaded semiconductor optical amplifier (SOA)-electroabsorption modulator (EAM) configuration for bi-directional 60-GHz-band radio-on-fiber (RoF) system applications SOA cross-gain modulation and photodetection in EAM are used for frequency up-conversion, and EAM nonlinearity is used for frequency down-conversion In our scheme, both 60-GHz local-oscillator (LO) signals and IF signals are optically transmitted from a central station to base stations We characterize the dependence of frequency up/down-conversion efficiencies on EAM bias and optical LO power For frequency up-conversion, maximum conversion gain of approximately 8 dB was obtained and, for frequency down-conversion, more than approximately 18-dB conversion loss was measured Utilizing this frequency up/down converter, we demonstrate a 60-GHz bi-directional RoF link Optically transmitted downlink 10-Mb/s quadrature phase-shift keying (QPSK) data at 100-MHz IF are frequency up-converted to the 60-GHz band at the base station, and uplink 10-Mb/s QPSK data in the 60-GHz band are frequency down-converted to 150-MHz IF and transmitted to the central station In addition, the dependence of error vector magnitudes on IF signal power and wavelength is investigated

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SOA-EAM frequency up/down-converters for 60-GHz bi-directional radio-on-fiber systems

In this letter, a novel approach to compensate for the dispersion-induced power penalty in externally modulated 1550-nm analog fiber-optic systems is presented. The principle is based on the use of fiber-induced self-phase modulation to generate a chirp distortion effect opposite to that induced by the chromatic dispersion. A significant reduction of chromatic dispersion effects is achieved, which results in augmenting the frequency-length product of the fiber-optic link. It is theoretically and experimentally shown how this dispersion compensation may be controlled by means of the optical power injected to the optical fiber.

On the use of fiber-induced self-phase modulation to reduce chromatic dispersion effects in microwave/millimeter-wave optical systems

A closed expression of the frequency transfer function nonlinear single-mode optical fibers in microwave optical systems is provided. This expression takes into account both chromatic dispersion and self-phase modulation effects and is valid for optical double-sideband modulation, optical single sideband modulation, and chirped optical transmitters.

Frequency transfer function of dispersive and nonlinear single-mode optical fibers in microwave optical systems

The authors report theoretical and experimental results on the compensation of chromatic dispersion effects in microwave/millimeter-wave optical systems employing four-wave mixing (FWM) induced in dispersion shifted fiber (DSF). The technique is based on generating a phase-conjugated wave by means of FWM in a 12.7-km-long DSF placed at the midspan of the fiber-optic link. The experimental results show that by using FWM a complete compensation of the electrical carrier suppression effect in a 50-km standard single-mode-fiber optic link is achieved.

Compensation of chromatic dispersion effects in microwave/millimeter-wave optical systems using four-wave-mixing induced in dispersion-shifted fibers

A comparative study between optical single-sideband (SSB) modulation and chirped fiber gratings (CFG) as dispersion equalization techniques is carried out. The simulation results show that the dispersion-induced distortion arising in optical millimeter-wave multichannel systems can not be compensated by employing the SSB modulation approach. Furthermore, we have demonstrated that the use of a CFG significantly reduces (nearly 25 dB) the composite triple beat (CTB) distortion. An accurate design of the CFG however must be addressed because the equalized CTB levels are very sensitive to the nonideal characteristics of the CFG such as a deviation of the optimum group delay slope or ripples.

Comparison of optical single-sideband modulation and chirped fiber gratings as dispersion mitigating techniques in optical millimeter-wave multichannel systems

Compensation for composite second-order (CSO) distortion due to the interplay of fiber chromatic dispersion and self-phase modulation in externally modulated amplitude modulated subcarrier multiplexing (AM-SCM) systems employing optical-phase conjugation (OPC) is investigated. The simulation results show that significant reductions (>20 dB) of fiber-induced CSO may be achieved using OPC, which results in an increase of the transmission distance in AM-SCM video distribution systems operating near 1550 nm. In addition, a comparison with another compensation technique, the phase modulation technique, is established, showing that OPC is much less sensitive to variations in the fiber length. However it is also calculated that for practical implementation of the OPC-based compensation technique, 25 to 30 dB reduction in the amplified spontaneous emission (ASE) associated with the OPC will be required.

F. Ramos

Papers

On the use of fiber-induced self-phase modulation to reduce chromatic dispersion effects in microwave/millimeter-wave optical systems

Frequency transfer function of dispersive and nonlinear single-mode optical fibers in microwave optical systems

Compensation of chromatic dispersion effects in microwave/millimeter-wave optical systems using four-wave-mixing induced in dispersion-shifted fibers

Comparison of optical single-sideband modulation and chirped fiber gratings as dispersion mitigating techniques in optical millimeter-wave multichannel systems

Compensation for fiber-induced composite second-order distortion in externally modulated lightwave AM-SCM systems using optical-phase conjugation